Server Blacklisted

I have my server blacklisted and my customers with on hotmail.it, libero.it emails doesn't receive emails about registration on my websites what I must do to remove my server from your blacklist?

I post server IP to 

http://ipremoval.sms.symantec.com/lookup/

but it's still in your black list. It's the only blacklist that block mails from my server.

Americo